X(10-x)=20

Solution for X(10-x)=20 equation:

X(10-X)=20

We move all terms to the left:

X(10-X)-(20)=0

We add all the numbers together, and all the variables

X(-1X+10)-20=0

We multiply parentheses

-1X^2+10X-20=0

a = -1; b = 10; c = -20;
Δ = b2-4ac
Δ = 102-4·(-1)·(-20)
Δ = 20
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{20}=\sqrt{4*5}=\sqrt{4}*\sqrt{5}=2\sqrt{5}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(10)-2\sqrt{5}}{2*-1}=\frac{-10-2\sqrt{5}}{-2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(10)+2\sqrt{5}}{2*-1}=\frac{-10+2\sqrt{5}}{-2}
$